1. Field of the Invention
The present invention relates to an elevator braking device. In particular, it relates to a device designed as an emergency elevator brake which engages and prevents an elevator car from leaving an intended floor while the doors are open or under other emergency situation.
2. Description of Related Art
A wide variety of devices are movable in an up and down direction by means of a cable or wire rope. Elevator cars as well as buckets, harnesses and the like operate by means of a counterweighted motorized cable hoist means.
Elevators in particular are operated by suspending the car by one or more wire ropes. The wire rope then goes over a fulcrum, such as a traction sheave, and then down the other side of the fulcrum to a counterweight. Counterweighing an elevator car is useful in that less energy is necessary to move the elevator up and down in an elevator shaft. The elevator car, the fulcrum holding the cable, or both are motorized directly or indirectly usually through some form a geared mechanism. When the elevator is to stop at a particular floor, a brake is applied to stop and hold the elevator car in position for boarding and exiting.
Elevators have been equipped with brakes that are designed to act as an emergency backup in case of rapid decent of the elevator. For example, if the cable holding the car were to brake, the elevator car would rapidly descend to the bottom of the elevator shaft. Frequently, emergency brakes designed to grip 2 or more rails in the event of an elevator exceeding a predetermined speed have been included in elevator design.
There is a second emergency situation which exists during use of an elevator car. Because the counter weight of the elevator is usually heavier than the elevator car itself, a failure of the normal breaking device means the elevator car could easily over speed in an ascending manner. This could result in the car not remaining stationary at a given floor when the doors are open or cause the car to raise to the top of the shaft and remain stuck there. The code in the United States (and possibly soon in other countries) requires emergency brakes which engage in the event of such upward movement.
A number of methods are currently used to accomplish this task. Older methods include brakes applied to the fulcrum or traction sheave, to the hoisting ropes, the wire ropes, to the elevator car or to the counter weight via the counterweight guide rails. Brakes which include elements, such as hoses, tanks, air cylinders, compressors, or the like, are frequently subject to failure rendering the brake inoperative. One method of grabbing the wire rope is disclosed in U.S. Pat. No. 5,228,540 issued Jul. 20, 1993 to Glaser and assigned to the Hollister-Whitney Elevator corp. In their brake, there is a pair of brake shoes for grabbing the wire rope with a cam means connected directly to one of the shoes where the cam is a pivotal follower attached to the movable brake shoe moving across a stationary cam surface. The cam is activated by a compressed spring. While this design does work, it can stick in the open, non braking position if the cam shaft is broken. Further, since their cam is spring activated they rely on activation of a piston connected to the spring directly to work. Once again, if this brakes, the braking means will not operate.
Since such emergency brakes are mandated and must operate with the least amount of potential problems, it would be important to design an emergency brake for an elevator car system which overcomes some or all of the above problems with currently designed elevator brakes which can render them non operative.